Does anyone know a good site to look up trade histories? 

For example who the A's traded to the Cubs for Harden (I know this one, but if I wanted to look something like that up....)

I was trying to remember a bunch of trades and look at the end value of prospects received for ML players during a season, say for a playoff push (a la' CC to the Brew Crew last year).  Interested in seeing what kind of value guys bring in during different times of year and possibly getting some insight into which teams might scout other teams' farm systems the best.